The statements in this section may serve as a background to help understand the invention and its application and uses, but may not constitute prior art.
Compared with conventional power devices made of silicon (Si), Group III-Nitride (III-N) semiconductors possess many excellent electronic properties that enable the fabrication of modern power electronic devices and structures for use in a variety of applications. Silicon's limited critical electric field and relatively high resistance make currently available commercial power devices, circuits, and systems bulky, heavy, with further constraints on operating frequencies. On the other hand, higher critical electric field and higher electron density and mobility of III-N materials allow high-current, high-voltage, high-power and/or high-frequency performances of improved power transistors that are greatly desirable for advanced transportation systems, high-efficiency electricity generation and conversion systems, and energy delivery networks. Such systems rely on efficient converters to step-up or step-down electric voltages, and use power transistors capable of blocking large voltages and/or carrying large currents. For example, power transistors with blocking voltages of more than 500V are used in hybrid vehicles to convert DC power from batteries to AC power. Some other exemplary applications of power transistors include power supplies, automotive electronics, automated factory equipment, motor controls, traction motor drives, high voltage direct current (HVDC) electronics, lamp ballasts, telecommunication circuits and display drives.
Despite the enormous potential of III-N semiconductor devices for producing high-efficiency power electronics such as high power amplifiers and converters, achieving desired or projected performances is still difficult. For example, best-performing III-N transistors are generally of the depletion (normally-on) type with negative threshold voltages that require a negative gate bias to block any current flowing through. Enhancement mode (normally-off) operations often degrade device performances, and normally-off III-N transistors often have low threshold voltages and high off-state leakage currents. Cascode structures consisting of a normally-on (depletion mode) III-N transistor driven by a normally-off Si-based power MOSFET have recently been proposed and utilized to achieve normally-off behaviors, yet significant thermal and transient voltage issues still exist during high-frequency operations that limit the realizable performance of such devices.
Therefore, in view of the aforementioned practicalities and difficulties, there is an unsolved need for devices and circuits that enhance the performance of Group III-Nitride semiconductor structures.